This invention relates to generation of fast rise time acoustic waves and, more particularly, to an improved stylus for generation of fast rise time acoustic wave energy for use in position determining equipment.
Systems known as "acoustic digitizers" determine the position of a movable indicator by determining the travel time of sound between the indicator and known positions. In many acoustic digitizer applications, a hand-held stylus is utilized, much as a writing pen, to point to a particular position in a working area. The stylus tip is connected to a microswitch which is, in turn, coupled to circuitry that electrically generates a spark at or near the stylus tip. This results in shock wave acoustic energy which radiates from the region of the tip. Clocks are started in synchronism with the generation of the spark. When acoustic energy arrives at known locations (typically two or more spaced apart microphones), the clocks are respectively stopped and the times of propagation of the acoustic energy from the stylus tip to the microphones are used to automatically calculate the positional coordinates of the stylus tip, based on trigonometric relationships and the speed of sound in air.
An example of a prior art type of stylus for use in an acoustic digitizer is illustrated in U.S. Pat. No. 3,626,483. As described in that patent, a tubular housing contains a conductive ink cartridge having a ball point for writing. A microswitch is mounted at the rear of the ink cartridge. A wire runs through a passageway in the housing and terminates at a tungsten whisker that forms a spark gap with the tip of the cartridge. The microswitch controls application of a potential across the spark gap. A notched barium titanate collar is provided around the tip of the cartridge. The notched collar is used to obtain consistency in the direction of the spark.
Another prior art spark pen configuration utilizes a pair of parallel wires in a insulated ceramic carrier, the tips of the wires protruding from the ceramic to form a spark gap that is near the tip of the stylus. Application of a suitable potential across the wire pair generates a spark near the stylus tip.
The above-described types of sparkling styluses could stand improvement in the accuracy and consistency of the digitizer measurements produced by systems that employ such styluses. The exact position of the spark gap will affect the time of propagation of the acoustic energy to the system microphones. However, due to the nature of construction of the stylus tip and spark gap, the spark gap can be at different positions, depending upon the rotational orientation and the tilt of the stylus, even when the stylus point is at the same fixed position. Consider, for example, the above described sparking pen of U.S. Pat. No. 3,626,483. For a given position of the ink cartridge tip, the conductive whisker can be at any rotational orientation with respect to the cartridge axis, and the exact location of the spark will be determined by said rotational orientation. Some degree of consistency of measurement can be achieved by having the user always have the sparking pen at approximately the same rotational orientation, such as by providing an indicating dot on the spark pen or by having the user always have the whisker point in the same direction. In addition to being inconvenient, this expedient also leaves room for substantial error. The same drawbacks will be present in a sparking stylus that uses a wire pair with a spark gap on the side of the ink cartridge or other stylus.
Another source of possible inconsistency or error in the prior art sparking styluses can arise from different tilt angles of the stylus. Since the spark gap is at a significantly higher elevation than the stylus tip which contacts the working surface, a change in the degree of tilt of the stylus will change the effective spark position. Again, it is possible to somewhat control this inconsistency or error by having the user always employ approximately the same stylus tilt but, as above noted, this tends to be inconvenient and leaves room for error.
It is an object of the present invention to provide an improved sparking stylus that substantially overcomes the types of problems described above, and permits more convenient, more consistent, and more accurate operation of acoustic digitizer systems.